The RNA-binding protein RNPC1/Rbm38 is a p53 target and repressor of p53 mRNA translation. While the p53–RNPC1 loop is critical for modulating p53 tumor suppression, it is unclear how the loop is regulated. Here, Zhang et al. show that glycogen synthase kinase 3 regulates p53 mRNA translation via phosphorylation of RNPC1. Surprisingly, they find that phosphorylation of a specific site converts RNPC1 from a repressor to an activator of p53. This study may have important therapeutic implications for the modulation of p53 activity in cancer cells.